Process for grinding rubber



United States Patent O 3,199,565 PROCESS FUR GRINDENG RUEEER Wili am 1V. .iayne, in, Basking Ridge, Ml, assignor to Union Carbide "Corporation, a corporation of New York No Draw/in". Filed Aug. 24, 1962, filer. No. 219,105 6 Claims. (6i. 24-12Z) The present invention relates to an improved method of grinding materials which are characterized by tackiness, elasticity, and a tendency to agglomerate. More particularly, it relates to an improved method of grinding natural or synthetic rubbers which are characterized by tackiness, elasticity, and a tendency to agglomerate.

For many uses in industry it has been found desirable to finely divide materials which are soft and cohesive by nature. Such fine division has been desirable or necessary in order to facilitate adequate dispersion Within the material of various agents which are desirable to have present in the ultimate product. Illustrative of such agents are vulcanizing agents, antioxidants, heat stabilizers, pigments, and the like. When the material has been adequately ground into small particles the inclusion of the treating agents becomes relatively simple due to the increased surface area of the granular material.

In addition to the greater facility of incorporating additives, it has also been found desirable to finely divide these materials as they are much more readily handled and transferred when they are in a smaller particular form.

Grinding of soft materials is generally accomplished in a mechanical grinder, cutter, chopper, granulator or the like which utilizes cutting blades to divide the material. Preferred generally are those machines which utilize rotary cutter blades, or knives.

Because of the inherent characteristics of sticky, agglomerating material, it has been almost impossible to grind, chop, or granulate such materials in standard mechanical grinders, granulators, or cutters without employing special techniques, because the material either sticks to and gathers about the cutter blades or re-agglomerates after grinding. The happening of either of these occurrences makes the grinding operation futile and impractical. The machine has to be periodically stopped while the cutter blades are cleaned of the material, if it gathers about the blades, or ceased entirely, if the material re-agglomerates as fast as it is granulated.

In the past attempts have been made to overcome the difiiculty in granulating such materials by various techniques.

One such method has been to freeze the material prior to or during granulation. This is generally accomplished by adding Dry Ice to the grinding batch. The Dry Ice hardens the surface of the material, reducing the tackiness and permitting the blades to fracture and cut the material. This method does however have several serious drawbacks; the Dry Ice also cools the cutter blades and makes them more brittle which can result in blade fracture; the Dry Ice is expensive to both obtain and maintain which reduces the efficiency of the operation; and after the material has been warmed to room temperature it tends to re-agglomerate.

A second method of grinding these materials has been through the use of machines equipped with special cutter blades which break up the material followed by dusting with talc. This procedure is effective in that the material will not re-agglomerate but it is restrictive in that the talc can interfere with the end use of the material. In many cases the end product when heated results in an undesirable sintered product. This method is also hampered by the additional expense of acquiring special cutters which also reduces the efiiciency of the opera- 3,13%,565 Patented June 22, 1955 tion, and it is totally impractical where only small batches of such material is to be ground as the added expense of special equipment could never be justified. For this reason the second method described above is limited to those manufacturers who handle large quantities of such materials. Until now no method had been found which simply and efficiently overcame the deficiencies of the prior art methods.

In accordance with the present invention, it has been found that the inclusion of a small amount of polyolefin fines to the material being ground eifectively prevents cutter buildup of the material and material agglomeration after cutting without interfering with the subsequent use of the material.

The polyolefin fines which are useful in the present invention are those of polyethylene and polypropylene having an average molecular weight in excess of 15,000. Polyethylene has been found to be preferable, however, in that it has a lubricating effect on the cutters which increases the time which the blade can be used without resharpening. The polyolefin fines used in accordance with the present invention should have a diameter at least sufiicient to pass through a standard mesh screen of at least 40 and preferably of from to 200 mesh.

The amount of polyolefin fines which should be used is from: 1 part polyolefin to 15 parts material to be ground to 1 part polyolefin to 30 parts material to be ground, all parts by weight. It is preferred to use 1 part polyolefin to 30 parts material as the smaller amount of any additive which is foreign to the material granulated produces a purer ult mate material for its end use.

While the olefin fines can be added batchwise to the cutter simultaneously with or subsequent to adding the material to be granulated, it has been found desirable to continuously add the fines during the granulating operation as a smaller amount of fines is required for a satisfactory operation. The fines should never be charged to the cutter prior to the material to be ground or the prior art deficiencies will be realized.

Illustrative of the materials which are characterized by stickiness, elasticity and a tendency to agglomerate, to which the present invention is applicable, are: the natural rubbers such as gum rubber, India rubber, para rubber, guttapercha and the like and the synthetic rubbers such as butadiene-styrene copolymer, butadieneacrylonitrile copolymer, ethylene-propylene copolymer, chloroprene (neoprene), polybutadiene, polyisobutylene, polyisoprene, butyl rubber and the like, all in unvulcanized form.

The initial size of the material to be ground is not critical and is generally limited only by the operating capacity of the grinding mill or granulator. While generally the material to be ground is in the form of a slab, large particles or pellets may be ground with equal facility.

The temperature of the material to be ground should be held below the rubbery transition temperature and above its freezing temperature. The preferred grinding temperature is room temperature i.e. approximately 25 C.

Additional advantages attained through the use of the present invention are:

(1) the ability to utilize low cost conventional cutters,

(2) the elimination of the need for Dry-Ice chilling of the rubber prior to granulating,

(3) the ground product is non-sintering, and

(4) the ground product is free of inorganic contaminant.

Example Bale rubber was cut into slabs having a size of about 2 inches by 4 inches by 8 inches. These slabs were then fed to a parallel-knife cutter in a normal manner but simultaneous with the charging of each piece of rubber was added about 30 grams of polyethylene fines having an average mesh diameter of 140. The grinding operation was continued for a period of 35 minutes. At the end of this time the product and the cutter blades were examined. The product was in the form of non-tacky rubber pellets having an average size of A" to V8. An examination of the cutter blades revealed that they were free of rubber accumulation.

This experiment was repeated exactly without adding the fines. After a period of 4 minutes the product and blades were examined. The product was in the form of re-agglomerating particles which were tacky. An examination of the blades showed that they were completely incasecl in tacky-rubber.

What is claimed is:

1. In the method of grinding materials which are characterized by tackiness, elasticity and a tendency to agglomerate, in a mechanical cutter or grinder which utilizes cutting blades, the improvement of adding polyolefin fines having a diameter at least sufficient to pass through a standard mesh screen of at least 40 mesh, simultaneously with or subsequent to the addition of the material to be ground to the cutter or grinder, said polyolefin selected from the group of polyethylenes and polypropylenes having an average molecular weight in excess of 15,000.

2. The method of claim 1 wherein the polyolefin fines are polyethylene fines.

3. The method of claim 1 wherein the polyolefin fines are polypropylene fines.

4. The method of claim 1 wherein the polyolefin fines have a diameter suflicient to pass through a standard mesh screen of from about 100 to 200 mesh.

5. The method of claim 1 wherein the polyolefin fines are added in an amount of from 1 part olefin fines to 15 parts of material to be ground to 1 part olefin fines to 30 parts of material to be ground.

6. The method of claim 1 wherein the material to be ground is raw natural rubber.

References Cited by the Examiner UNITED STATES PATENTS 2,895,939 7/59 Stober et a1. 24l22 XR I. SPENCER OVERHOLSER, Primary Examiner. 

1. IN THE METHOD OF GRINDING MATERIALS WHICH ARE CHARACTERIZED BY TACKINESS, ELASTCITY AND A TENDENCY TO AGGLOMERATE, IN A MECHANICAL CUTTER OR GRINDER WHICH UTILIZES CUTTING BLADES, THE IMPROVEMENT OF ADDING POLYOLEFIN FINES HAVING A DIAMETER AT LEAST SUFFICIENT TO PASS THROUGH A STANDARD MESH SCREEN OF AT LEAST 40 MESH, SSIMULTANEOUSLY WITH OR SUBSEQUENT TO THE ADDITION OF THE MATERIAL TO BE GROUND TO THE CUTTER OR GRINDER, SAID POLYOLEFIN SELECTED FROM THE GROUP OF POLYETHYLENES AND POLYPROPYLENES HAVING AN AVERAGE MOLECULAR WEIGHT IN EXCESS OF 15,000. 